Experiments are proposed that investigate two novel and complementary approaches to labelling ensembles of physiologically active neurons in the olfactory system The primary technique is a molecular approach exploiting use of a proto-oncogene c-fos, which is expressed at low levels in most cells. It is based on the finding that diverse stimuli induce expression of Fos, the protein product of this gene, which can be detected histochemically. Experiments will determine if olfactory stimulation can induce c-fos expression in the somata of primary receptor cells, as well as in those of higher-order olfactory neurons. The second approach is based on the activity-induced endocytosis of fluorescent dyes into cellular processes at active synapses. Experiments will determine if perfusion of these dyes into the brain during olfactory stimulation selectively labels neuropil regions which are functionally active. In both instances, studies will determine the time course of c-fos induction and/or dye uptake and will examine the odor intensity and specificity required to produce this selective labelling. Compared to previously used ensemble-labelling techniques, either of these two methods has the capacity to provide superior Spatial and temporal resolution in labelling physiologically active neurons and would provide a powerful new way of investigating pattern discrimination in the olfactory system.